The present invention relates generally to cold rolled steel strip from which is made the core of an electric motor, and more particularly to steel strip which imparts to the core a relatively low core loss and a comparatively high peak permeability.
An electric motor is composed of a stator surrounding a rotor. The stator is composed of wire made from a relatively high conductivity material, such as copper, wound on a core composed of steel. The steel core of an electric motor is made up of laminations fabricated from cold rolled steel strip, typically composed of silicon-containing steel, and the steel laminations impart to the core properties known as core loss and peak permeability which affect the power loss in the motor. Core loss, as the name implies, reflects power loss in the core. Peak permeability generally reflects power loss in the winding around the core. Core loss is expressed as watts per pound (W/lb.) or watts per kilogram (W/kg.). Peak permeability is expressed as Gauss per Oersted (G/Oe.). Permeability may also be described in terms of relative permeability in which case it is expressed without units although the numbers would be the same as the numbers for the corresponding peak permeability. Core loss and peak permeability are both measured for the magnetic induction at which the core is intended to operate. Magnetic induction is expressed as Tesla (T) or kiloGauss (kG). A typical magnetic induction is 1.5 T (15 kG).
Thus, core loss reflects the power loss due to the core at a given magnetic induction, e.g., 1.5 T (15 kG), and peak permeability reflects the magnetizing current in the material of the core at that given induction. The higher the peak permeability, the lower the magnetizing current needed to achieve a given induction. In addition, the higher the peak permeability for a given induction, the lower the power loss in the winding. Winding loss plus core loss are both important factors which reduce the efficiency of the motor.
Core loss and peak permeability are inherent properties of the steel strip from which the core laminations are fabricated. Therefore, an aim in producing steel strip for use in making the core of an electric motor is to reduce the core loss and increase the peak permeability of that steel strip, both of which factors increase the efficiency of the motor. Both of these factors are affected by the composition and heat treatment of the strip.
Moreover, for a steel having a given composition and heat treatment, core loss increases with an increase in the thickness of the strip rolled from that steel. Thus, comparisons of core loss should be made on steel strips having comparable thicknesses. For example, assuming a core loss of 5.10 W/kg (2.30 W/lb.) at a strip thickness of 0.018 inches (0.46 mm.), if there is then an increase in thickness of 0.001 inch (0.0254 mm.), the core loss will increase typically at an estimated rate of about 0.22 W/kg (0.10 W/lb.).
The considerations described above are discussed in Rastogi U.S. Pat. No. 4,390,378 etitled "Method for Producing Medium Silicon Steel Electrical Lamination Strip", and the disclosure thereof is incorporated herein by reference.
The steel strip disclosed in U.S. Pat. No. 4,390,378 is what is known as a semi-processed steel strip. More particularly, the final desired magnetic properties (core loss and average peak permeability) are not present in the steel when it is shipped by the steel mill to the customer who stamps out the laminations and then subjects the laminations to a decarburizing anneal as a result of which the final desired magnetic properties are produced. The resulting laminations have a 1.5 T (15 kG) average core loss value less than about 5.1 W/kg (2.30 W/lb.), and average peak permeability more than about 1,800 G/Oe. for a sample thickness of about 0.018 inch (0.46 mm.). This is accomplished with a steel composition which includes 0.85-1.05 wt. % silicon and 0.20-0.30 wt. % aluminum.
A possible drawback to the use of a semi-processed steel of the type described above is that it requires a decarburizing anneal by the customer which may be considered undesirable. A decarburizing anneal involves relatively stringent annealing requirements and consumes significant amounts of energy, the annealing being conducted at a temperature in the range 760.degree.-843.degree. C. (1,400.degree.-1,550.degree. F.) for about 1-2 hours. Moreover, there are sub-surface oxidation problems associated with a decarburization anneal conducted at this stage of the manufacturing operation.
An expedient for obtaining a lamination having the magnetic properties discussed above, and without requiring the customer to conduct a decarburizing anneal, is to employ a steel having a greater silicon and/or aluminum content, e.g., a combined silicon plus aluminum content in the range of about 1.85-2.40 wt. %. In contrast, the semi-processed steel of U.S. Pat. No. 4,390,378 has a combined silicon and aluminum content no greater than about 1.25 wt. %.
The steels with the higher silicon plus aluminum content are fully processed steels on which the customer conducts no decarburization operation after stamping out the laminations, but these steels have their own drawbacks. The higher the silicon content, the lower the saturation magnetization and the lower the magnetic permeabilty at high induction (.gtoreq.1.5 T(15 kG)), and the greater the likelihood of cracking during reduction of the steel from a slab to a hot-rolled strip. The higher silicon content in the steel strip also reduces the life of the dies used to stamp out the laminations from the strip. As for aluminum, the higher the aluminum content, the greater the likelihood of producing a "dirty" steel, when employing conventional steel-making practices without vacuum degassing.
Thus, the prior art expedients for producing a steel lamination having the magnetic properties described above require either a relatively high silicon plus aluminum content, with its attendant drawbacks, or require the customer to employ a decarburization anneal in those instances where the silicon plus aluminum content is relatively low.